sand-dunes sprang into new and unexpected importance, from the fact that
they formed one of the principal hindrances to the free movement of
troops, and more especially of military motor transport, across the
desert. Every line of dunes of any considerable extent had now to be
carefully mapped and examined for possible car-passages through it; the
result was to add greatly to our knowledge of the distribution and
extent of these remarkable features of the Libyan Desert, especially in
the northern parts, where many long lines of dunes were found of which
the existence was previously unsuspected by geographers, though they are
familiar landmarks to the Bedouin of the region, and all except the
smallest bear Arabic names. After the cessation of hostilities, interest
in the distribution of dunes was maintained, because of the increasing
use of motor-cars in place of camels for transport across the desert;
and this circumstance has recently led to light being thrown on the
distribution of the dunes in those southern regions which lay outside
the field of operations during the war.
It is rather curious to note that although the sand-dunes form the
greatest obstacle to motor-cars in the desert, yet it is chiefly by
means of motor-cars that the true extent and distribution of the sand-
dunes has been ascertained. Dunes are the most difficult of all desert
features to map properly by ordinary reconnaissance methods with camel
transport. Their smooth outlines provide no points on which
intersections can be made, and no survey marks put on them will remain
in place for more than a few hours, or at most a few days; they occur
mostly in nearly level country, where it is impossible to find a station
whence they can be overlooked; the absence of shadows on them renders it
impossible to say whether one is looking at a single line of dunes, or
at several lines, miles apart, one behind the other in echelon. The only
sure way of mapping dunes is to traverse their sides along their entire
length, and this is impracticable with camels owing to the enormous
distances which would have to be covered without water. But with motor-
cars one can run alongside them at 40 kilometres an hour instead of the
camel’s 4, and their distribution can thus be rapidly and easily
ascertained.
[Illustration: _Prince Kemal el Din’s expedition of 1925 leaving Bir
Terfawi for the waterless journey of 250 miles to Owenat_]
[Illustration: _The well at Bir Terfawi in the palm clump_]
[Illustration: _Jebel Owenat from the south: the triple peak (5635 feet)
in centre_]
[Illustration: _The lake at Merga located by Prince Kemal el Din’s
expedition of 1925_]
On the map I have shown the distribution of sand-dunes as far as it is
known at the present day. A comparison of this latest map with that
given by Mr. Beadnell in his paper of 1910 will show how large is the
number of dunes discovered and mapped in more recent years; and it will
be noticed how pronounced is the constancy of the general direction of
all the lines in the Egyptian portion of the Libyan Desert. Though no
new lines of dunes comparable in extent with the great Abu Moharik belt
have been added to the map, some of the newly discovered lines are of
very considerable length, and they show an even more remarkable ratio of
length to breadth than the Abu Moharik belt; the Ramak dunes, for
instance, extend from near Moghara south-south-eastwards in a straight
line for more than 100 kilometres, with only a single small break, and
their width nowhere exceeds 1 kilometre. Moreover, the linear
arrangement in a direction from about 20° west of north to about 20°
east of south is even more pronounced in the newly discovered northern
dunes than it is in those previously known farther south. In the “great
sand sea” of Rohlfs, to the west of Dakhla and Farafra, the same
directional arrangement of the individual lines of dunes is very
noticeable, at least in the part near “Regenfeld” where I have
penetrated it; and I found the same general direction to hold for most
of the lines of dunes which I crossed on the way from Terfawi to Owenat.
During the war I received a number of reports of dunes extending nearly
east and west, often in curved lines; but investigations on the spot
showed practically all these reports to be erroneous, the commonest
mistake having been that of sighting different lines of dunes from
different places under the impression that they were a single one.
The general south-south-easterly direction which is so marked a feature
of the lines of dunes in the Egyptian portion of the Libyan Desert is
not, however, preserved in the extreme south-west of the country. The
line of dunes which crosses the Egypt-Sudan frontier in longitude 25°
40′ has a direction of about 30° _west_ of south, and the dunes which
extend from near Gebel Arkenu to the west of Gebel Owenat run about 38°
west of south. From a point about 80 kilometres to the south-west of
Gebel Owenat, the dune-lines make a still further distinct bend to the
west, changing their direction by some 20° to about 58° west of south,
and this latter direction is maintained by the dunes in the
neighbourhood of Sarra Well.[46] According to the maps of Rohlfs and
Mrs. Forbes, the dunes in the sandy tract to the north-west of Kufra
follow approximately the same bearing as those near Sarra. The
suggestion has been made that this swinging round of the direction of
the dunes in the south-western part of the Libyan Desert may be due to a
deflection of the prevailing wind by the mountain-masses. It is almost
certainly caused by differences in the prevailing winds—in fact, the
dune-lines probably afford a very exact index to the general wind-
direction in the areas where they occur—but I am inclined to think that
the regional distribution of atmospheric pressure, rather than local
deflection by mountain masses, is the cause of the differences of wind-
direction. The dunes, especially those near Sarra and Kufra, extend into
localities which seem to me to be too distant from the mountains for the
influence of the latter on the prevailing winds to afford a satisfactory
explanation.[47]
In certain areas, especially in the south, as for instance on the Arbain
road from Kharga to Sheb, around Pottery Hill, and in the region between
Owenat and Erdi, there are sand-covered tracts which can hardly be
mapped as dunes, because there are no very marked crests. These tracts
are easily crossed by camels, but give much trouble with cars, and would
probably be classed as dunes by car-drivers; but they are really only
vast undulating fields of drifted sand. On the old Darb el Arbain slave
road between Kharga and Sheb, one travels for more than 100 miles over
drifts of this kind. No footprint of camel marks the track, footprints
being obliterated in an hour or two. But one is never in doubt about the
way, for it is indicated by the skeletons of countless camels which have
perished on the weary march.
As regards the relation between dunes and the relief of the ground,
there is a gradual rise in the general level of the country from north
to south of about 1 in 1000, so that the southern ends of most of the
Egyptian lines of dunes lie at higher altitudes than the northern ends;
but I am fairly certain that this slight general inclination has nothing
to do with dune-formation. The straightest and cleanest-cut lines of
dunes are found in the flattest and most open parts of the desert, over
which the wind can sweep without obstruction. When a line of dunes
encounters in its southward progression a sudden fall in the ground-
level, as for instance at the north end of Kharga Oasis and to the east
of Pottery Hill, it usually breaks up; sometimes, as in Kharga, to
resume its course as dunes in more open formation, and at other times,
as near Pottery Hill, to spread out into a rolling plain of drifted
sand. It is not uncommon for the south end of a line of dunes to split
up, even on level ground, and the same can be seen, though less often,
at the northern ends; but this is probably merely because the dunes are
lower near the ends of the lines, and the width of the individual dunes
is consequently less.
The great lines of dunes are probably extending southwards at a very
slow rate, under the action of the prevailing wind. In this connection
it is interesting to note that Arabs always speak of the south end of a
line of dunes as its head, and of the north end as its tail; just the
opposite of what one would instinctively call them from merely looking
at the map, but correct if the dunes are known to have a southward
progression. Mr. Beadnell found the average rate of progression of
isolated small dunes in Kharga to be 15 metres per annum; but he remarks
that large dunes move more slowly than small ones, owing to the greater
mass of material to be transported. It is interesting to make a rough
guess at the antiquity of the dunes from the length of certain of the
longer lines and an estimated rate of progression. The Abu Moharik belt,
for instance, has a total length of about 350 kilometres; hence, if its
tail has remained stationary and its head has advanced southwards at an
average rate of 10 metres a year, some 35,000 years must have been
occupied in its formation.
As regards the height of the dunes, the only one I have carefully
measured is that close west of Rohlfs’ camp at Regenfeld, which I found
to be exactly 30 metres. Some of the dunes in the Abu Moharik and Ramak
belts are considerably higher; but I doubt if any of the Libyan dunes
rise much over 50 metres above ground-level, except in the “great sand
sea” which commences some 14 kilometres to the north-west of Regenfeld,
where Jordan estimated them to attain 100 metres or more.[48] The top of
a high dune to the south of Melfa Oasis was found by triangulation to be
172 metres above sea; the level of the ground at its foot is unknown,
but is probably not very much above the sea-level, so that Jordan’s
estimate of the height of some of the dunes in the “sand sea” is
probably by no means an exaggerated one.
Much has been written about eddies in connection with dune-formation.
The only observations I have made in this connection have been when
endeavouring to place temporary marks on dunes to form survey-points;
and the observations seem to show that if eddies are artificially
created at the top of a dune, a rapid lowering of the dune-crest
results. At first I used to thrust a walking-stick or ranging-rod some 2
or 3 feet into the dune-crest. The stick or rod, though quite firmly
fixed at first, was always found after a few hours lying halfway down
the dune-slope. The same thing happened with tripods firmly pressed down
into the crest. When I was carrying out a little triangulation to
determine the position of Rohlfs’ cairn at Regenfeld in 1924, I had
occasion to place a mark on a high dune, and I used an empty 4-gallon
petrol tin filled with sand, bedding it well into the dune; but the next
day it had disappeared from the crest, and was found, as the other
things had been, halfway down the slope. These observations remind me of
Mr. Barclay’s description of the method of dealing with sand-
accumulations in the Peruvian Desert,[49] where, as soon as dunes appear
and threaten to obstruct a railway, the local inhabitants turn out and
scatter pebbles and stones on the dunes, and very soon the dunes are
gone, having been carried away owing to eddies produced by the wind
around the stones. This device for removing incipient dunes might, I
think, be tried along the Kharga Railway, where the screens put up at
Mr. Harding King’s suggestion have not been entirely successful in
preventing the encroachment of sand on the line. I am the more disposed
to think an experiment of this kind might succeed, because I have
noticed that at the gap in the Ramak dunes (discovered by Col. Partridge
and known during the war as “Partridge Gap”) there are great lumps of
silicified wood, which may have led to the gap being formed.
There are two other physical characteristics of dunes which I do not
remember having seen mentioned. One of these is their low thermal
conductivity. On a hot day, one has only to thrust one’s hand a few
inches into a dune to encounter cool sand, while on a bitterly cold day
one can warm one’s hands by the same action; thus while the diurnal
variation of temperature of the surface of a dune is frequently very
great, it is almost insensible at quite a shallow depth in the interior.
It follows that diurnal expansion and contraction of the grains
composing the dune must be confined to the superficial layers, and the
movements thereby caused may materially assist in consolidating the
dune, by causing a “packing” of the grains. The other characteristic I
have noticed is that the surfaces of the dunes always appear to be
harder underfoot in the cool of the early morning than in the heat of
the day. This may, of course, be an illusion owing to one’s being less
sensitive to fatigue in the early morning, when one’s body has been
refreshed by sleep, and when the air-temperature is comfortable; but I
have often fancied there is more in it than that, and it may easily be
that the “packing” which has taken place owing to the considerable fall
of temperature in the night, to say nothing of the action of dew, may
have caused a real compacting of the surface layers.
In his paper of 1910, Mr. Beadnell considered that the sand composing
the dunes must be derived from arenaceous formations to the north, and
he made special mention of the need for a careful survey of the region
in which the dunes originate. A reference to the map will show how
correct was Mr. Beadnell’s surmise, at least in the case of the Egyptian
dunes which he had studied. The dunes originate in the great depressions
which stretch from Siwa to the Wadi Natrun, where, as Mr. Beadnell
inferred, the supply of arenaceous material from the loosely compacted
Miocene and Oligocene beds is almost inexhaustible. We have here one of
the clearest possible examples of the powerful action of the wind as an
excavating and transporting agent. The Qattara depression has been
excavated, largely by the wind, to a depth of over 130 metres below sea-
level, and the excavated material has been carried southwards, some of
it for nearly 1000 kilometres in distance and more than 500 metres in
height, to form the dunes. We see not only the excavated hollow, but
also the transported and piled-up arenaceous material removed from it;
and the total quantity of rock thus removed and re-deposited must amount
to hundreds of cubic kilometres.[50]
It is noticeable, however, that to the north of the “great sand sea” of
Rohlfs, which extends for an unknown distance westward, the known
depressions are much shallower; and yet the total quantity of sand in
this region is probably greater than that of all the more easterly dunes
put together. The country between Jarabub and Jalo is however almost
unexplored, though it is known to be very sandy and to be the theatre of
violent sandstorms. Whether similar deep depressions to that of Qattara
have been excavated along, or to the north of, the Jarabub-Jalo road, or
whether the area of loosely compacted arenaceous rocks exposed there is
so enormous as to have yielded such a quantity of sand without any very
deep depressions being formed, is a point that can only be cleared up by
further exploration in Cyrenaica.
We now come to the most puzzling of all the questions connected with the
sand-dunes, namely, why do they persist as sharply defined long narrow
lines in certain places, while the ground elsewhere is almost absolutely
free from sand? As Mr. Harding King puts it, the dunes appear to have a
curious power of collecting all the sand in the neighbourhood.[51] The
sheltering effect of ripples in the dunes, and a “shepherding” effect of
winds a point or two off the normal, have been suggested as possibly
affording some explanation of this apparent collecting-power,[52] but
neither of these suggestions appears to be adequate. I think a more
direct explanation is possible; namely, that the dunes really _have_ the
power of attracting sand from the air, and that this power is due to the
well-known law of attraction between a conducting surface at zero
potential and an electrically charged body. That air-borne sand-grains
in the neighbourhood of a dune may carry electric charges was shown by
Mr. Harding King’s experiments, recorded in his paper of 1916, and I
have since found sand-grains to carry electric charges of high voltage
during sandstorms in Cairo. As to how the grains become charged with
electricity, it can hardly be due to friction against each other, for
the sand is remarkably uniform in composition, consisting generally of
rounded grains of almost pure silica coated with a thin translucent film
of ferric oxide,[53] and for the production of frictional electricity it
is generally regarded as necessary that the two bodies which are rubbed
together should be of dissimilar materials;[54] nor is it likely that
they are electrified by friction with the air, for they travel with it.
But if the particles are carried into an upper air-stratum, they will
gradually acquire its potential by conduction; and the ordinary
potential gradient in the air in Egypt is over 100 volts per metre of
height.[55] Hence a sand-particle carried along for some time in the air
at a height of only 20 metres above the ground may become charged to 7
electro-static units of potential, or 2100 volts; and on approaching a
dune rising to about that height it will be attracted to the dune, which
being in connection with the earth is at zero potential. The attraction
will be very small unless the sand-grain approaches very closely to the
surface of the dune; from some calculations I have made, a spherical
grain of silica half a millimetre in diameter charged to a potential of
2100 volts will not be attracted with a force equal to half its own
weight until it approaches within a centimetre of the conducting
surface, and the grain will have to approach within 7 millimetres of the
surface before the attraction is equal to the weight of the grain. But
the important thing to note is that the attraction is independent both
of the sign of the charge and of the direction of the wind which carries
the particle. Another way of looking at the matter is to consider the
probable distribution of the equipotential surfaces in the air about a
dune, as in the diagram below, which represents a section taken
perpendicular to the direction of the wind and to the axis of the line
of dunes:
[Illustration: _Diagram showing suggested distribution of equipotential
surfaces and directions of lines of force in the neighbourhood of a
sand-dune_]
The equipotential surfaces, shown by dotted lines, will be squeezed
together over the dune, and the electric forces, shown by full lines and
arrows, being everywhere normal to those surfaces, will converge to the
dune. Thus the dune will attract any particles carried by the wind and
charged by conduction to the potential of the air conveying them. If the
wind changes its direction, the attraction will still persist unaltered.
Another point worth mention is that a sand-dune may be far more
effective in its attraction of sand than a rocky ridge of the same size
and shape would be; for when the electrified grains approach the surface
of the dune, uncharged grains can rise from the dune to meet them;
possibly some of the dancing of sand-grains which is frequently
observable along the crest of a dune during a wind may be due to this
cause.[56] During violent sandstorms, particles of sand may be whirled
aloft and kept suspended at high altitudes for a period long enough for
them to acquire very high potentials, and a sudden cessation of the
storm may cause them to fall so rapidly that much of the charge may be
retained; in which case, of course, electrical attraction may cause a
very considerable deviation from the purely gravitational paths of the
falling particles, the deviation being always towards the dune. Since
the closeness of the equipotential surfaces and the mobility of the
sand-grains composing the dune are greatest along the dune-crest, the
maximum rate of deposition will be at the top of the dune, which is also
the place where the wind exerts its greatest action in conveying the
sand of the dunes southward. That the long axes of the lines of dunes do
really correspond with the direction of the prevailing wind is certain,
both from Mr. Beadnell’s observations of wind-direction[57] and from the
fact that the scoring of the limestones in the Libyan Desert by wind-
borne sand coincides in direction with the lines of dunes.[58] Thus I
suggest that while the extension of the lines of dunes southwards is
purely the result of the prevailing north-north-west wind, their clean-
cut character and narrow width is the consequence of lateral attraction
by the dunes themselves on the flying electrified particles of sand.
If the hypothesis I have ventured to put forward is regarded as likely
to furnish the true explanation for the formation of the lines of dunes,
I hope that someone possessing the necessary skill in electrostatic
measurement will put it to a crucial test, by carrying out observations
on and near the dunes themselves. If a number of flying sand-grains
could be caught from different levels in an insulated metallic receiver
connected to an electrometer, the readings of the electrometer would
enable the potentials to which the grains were charged at different
levels to be calculated, provided the electrostatic capacity of the
receiving system and the number and diameter of the grains giving up
their charge were measured. Allowance would, of course, have to be made
for any electrification of the receiver by the air, or by uncaught sand-
grains, as well as for losses by imperfect insulation of the collecting
system. In a preliminary experiment of this kind which I made in Cairo
during a sandstorm, leakage from the electrometer was found to be so
great as to vitiate any attempts at accurate measurement; but I think
this particular difficulty could be overcome with a specially designed
apparatus, or that at least the leakage could be measured and allowed
for. The taking of observations of any kind in dune areas during windy
weather is a most difficult and trying operation, and commonly means, in
addition, a sojourn of some weeks in isolated and otherwise
uninteresting areas. But the advent of the motor-car has so greatly
facilitated communication in the deserts that difficulties of access are
now not so great as they used to be; the south end of the 35-kilometre
line of the Kattania Dunes, for instance, about 90 kilometres west-
south-west of Cairo, which would form an excellent site for detailed
observations, can now be reached by a four or five hours’ car journey
from Cairo instead of the several days’ journey by camel which was
formerly necessary.
12. _The Distribution of Stone Implements._
I have not made any special study of stone implements, nor have I been
able to pay any considerable attention to their collection during my
journeys, having usually found my time very fully taken up with other
matters. I have, however, been much struck by the wide distribution of
stone implements in the desert. I have found them, for instance, not
only in Siwa Oasis and near the wells of Abu Mungar and Sheb, but also
on the plateau between Baharia and Farafra, on the open desert between
Terfawi and Owenat, and to the south-west of Owenat near the Anglo-
French boundary. This wide range of occurrence, coupled with the finds
of Schweinfurth and others on the plateaux nearer the Nile, inclines me
to think that there is scarcely any part of the Libyan Desert in which
stone implements might not be found by an expedition which would make
the collection and study of them one of its main objects. The likeliest
places in which to search (besides the neighbourhood of old wells and
springs) would appear to be the shores of the various lakes and around
the feet of hills affording shelter from wind and sun; for it is in such
localities that I have most usually come across implements and pottery.
Grinding-stones, often with a sort of stone rolling-pin, unpolished
celts, knives, and arrow-heads are the principal forms of implement I
have met with. Many of the grinding-stones must have been carried for a
considerable distance, for they are made out of rocks which do not
naturally occur in the localities in which they are found.